This invention pertains to a method and apparatus for the heat treatment of material and, more particularly, to a method and apparatus for the treatment of material by immersion of the material in a turbulent liquid interspersed with a plurality of jets of hot gas.
Certain plastic materials, notably polyethylene terephthlate (PET) and the like are preferably commercially used in a state of semi-crystallinity. Typical thermoplastic materials which can achieve this state have relative high melting points and are characterized by having crystalline regions interspersed among amorphous regions. To obtain a semi-crystalline state with such thermoplastic materials, sustained heating of the materials at a predetermined temperature range is required. The temperature level and duration of heating needed to reach a desired level of crystallinity depends, inter alia, upon a number of parameters such as the type of material and particle size as it is well recognized that some polymers crystallize much faster than others at the same temperature levels. It has been additionally recognized that crystallinity occurs at faster rates when particles of the material as opposed to bulk material are treated. For purposes of this description the words "particle" or "particles" will be used to describe the material being processed in particle form, but the use of such words should not be construed as limiting with respect to specific dimensions of the material being treated.
A typical industrial process for crystallizing plastic materials uses hot air distributed over material generally comminuted into small pieces and arranged in thin layers so as to allow as much exposure as possible to the air. The temperature of the material is raised to a level above its glass transition temperature which is well below its melting point. At these levels, the crystalline regions of the material are unaffected but the amorphous regions become soft. Crystallization is initiated at various locations within the amorphous regions and semicrystalline solid spherulites start growing about nuclei. Completion of the crystallinity process is basically completed when the whole material, i.e., each particle, has been converted to a spherulite semicrystalline solid.
A practical problem with the hot air process for crystallizing material, however, is the length of time needed to reach the desired state of crystallinity. Heating periods often exceed an hour or more which is not compatible with continuous processing. Additionally the material becomes tack during heating, resulting in significant handling difficulties.
A recent innovation developed to overcome the long processing time has been the use of high energy radio frequency waves to couple with thin layers of thermoplastic material such as PET on a moving conveyor belt. The PET is heated to temperatures of about 325.degree. F. and undergoes crystallization while on the moving belt. Because the PET is placed in thin layers the tackiness is less than in the hot air processes. Tackiness is not eliminated, however, since the thermoplastic particles remain in abutting relationships with each other throughout the treatment.
Thus, it is desirable to have an apparatus and method which could treat the thermoplastic materials so as to achieve the desired state of crystallinity in a short period of time without the handling problems associated with the tackiness of the material.